#box{
    width: 750px;
    height: 425px;
    border: 1px solid #cccccc;
    margin: 50px auto;
    padding: 5px;
    position: relative;
    overflow: hidden;
    margin-left: 23%;
}
#pic{
    /*display: block;*/
    width: 750px;
    height: 425px;
    position: relative;
    z-index: 1;
}
#pic li{
    position: absolute;
    left: 0;
    top: 0;
    /*透明度*/
    opacity: 0;
    z-index: 1;
    /*过度*/
    transition: opacity 0.2s ease-out;
}
#pic li.active{
    opacity: 1;
    /*优先级*/
    z-index: 2;
}
#pic img{
    display: block;
    width: 750px;
    height: 425px;
}
#dot{
    position: absolute;
    left: 50%;
    bottom: 20px;
    transform: translateX(-50%);
    z-index: 2;
}
#dot li{
    width: 10px;
    height: 10px;
    background: rgba(255,255,255,0.4);
    text-indent: 9999px;
    float: left;
    margin: 0 5px;
    cursor: pointer;
}
#dot li.active{
    width: 15px;
    background: rgba(255,255,255,1);
}

#dot ol,li{
    list-style-type: none;
}